The present invention relates to a method of manufacturing a semiconductor device, particularly, to a method of forming a solder bump electrode.
FIGS. 1A to 1D collectively show a conventional method of forming a bump electrode for a semiconductor device. As shown in the drawings, a pad electrode 1 made of, for example, aluminum is formed on an insulating film 12 covering the surface of a semiconductor substrate 3, followed by depositing an insulating protective film 2 on the entire surface. Then, that portion of the insulating protective film 2 which is positioned on the pad electrode 1 is selectively removed to form an opening exposing the pad electrode 1 to the outside.
Then, metal layers 4, which act as current supply layers in the solder plating step and as barrier layers after the solder plating step, are consecutively formed by a sputtering method on the entire surface, as shown in FIG. 1A. In general, the metal layers 4 are of a laminate structure consisting of three layers 4a, 4b and 4c. The lowermost layer 4a, which is made of, for example, Ti, is in direct contact with the pad electrode 1, i.e., Al layer, to form a strong adhesive bond with the Al layer. The intermediate layer 4b, which is made of Ni or Cu, acts as a barrier layer. Further, the uppermost layer 4c, which is made of a noble metal such as Pd or Pt, serves to form a strong adhesive bond with a solder layer which is to be formed thereon in the subsequent step and also acts as an anti-oxidation film.
In the next step, the entire surface is coated with a resist layer 5, followed by patterning the resist layer 5 by means of the ordinary photolithography technology so as to form an opening above the pad electrode 1, as shown in FIG. 1B. Then, an electroplating of solder is applied to form a solder bump 6 in the opening positioned above the pad electrode 1, as shown in FIG. 1C. After formation of the solder bump 6, the resist layer 5 is removed. Then, the metal layers 4 are etched using the solder bump 6 as a mask, as shown in FIG. 1D, thereby to prepare a semiconductor device comprising a solder bump electrode.
In the conventional method outlined above, the semiconductor device including the resist layer 5 is dipped in the plating bath in the step of forming the solder bump by the electroplating method. In general, the plating bath is strongly acidic, with the result that the resist layer 5 is partly dissolved during the plating step into the plating bath to form organic impurities. It follows that it is impossible to maintain as desired the composition of the plating bath. It is most important to control the composition of the plating bath in the electroplating. However, it is very difficult to maintain as desired the composition of the plating bath in the conventional method because organic impurities unavoidably enter the plating bath, as pointed out above.
Further, the uppermost metal layer 4c is made of a noble metal such as Pd or Pt, and the intermediate metal layer 4b is made of, for example, Ni or Cu in the conventional technique as already pointed out. What should be noted is that it is difficult to remove Pd, Pt, Au, Ni, Cu, etc. by a dry etching in the etching step for removing the metal layers 4 after formation of the solder bump 6. Therefore, the metal layers 4 are removed by a wet etching using an etching solution exhibiting an oxidizing power in the conventional method. However, each of Pb and Sn constituting the main components of the solder tend to be corroded easily by the etching solution exhibiting an oxidizing power. It follows that the solder bump 6 is also damaged in the etching step for removing the metal layers 4, leading to a low yield and a low reliability of the resultant semiconductor device.